In 2005, BASF Corporation won the Presidential Green Chemistry Challenge Award in the Greener Reaction Conditions category for their urethane acrylate oligomer primer system. Among many others, this system has the notable environmental advantages of low energy consumption and low VOC emissions. The purpose of this page is to summarize the “life” of a typical urethane acrylate oligomer, like the one produced by BASF, from start to finish in order to better understand how “green” the system actually is. Part I.My Parents (a.k.a. Raw Materials) (Principles Implicated: 2 & 4)
As with most things on the planet Earth, my life begins with my parents; the stuff that went into my creation.In my simplest form, I am made up of three main components: a di- or tri- functional polyol, a difunctional isocyanate, and another hydroxy containing compound which also has the reactive acrylate functionality.This third component is sometimes called a capping agent. Polyols are rather innocuous, non-hazardous starting materials some of which occur in nature, such as castor oil and sucrose, or can be derived from renewable vegetable oils.They can be aliphatic, aromatic, linear, cyclic, polyester based or polyether based, or any combination of the above.These various polyol characteristics that are inherited by urethane acrylate oligomers leading to enormous versatility in physical properties. My polyol parent is 1,4 butanediol. Here's a picture.
Unlike polyols, isocyanates are quite a hazardous bunch causing asthma in humans due to the high reactivity of the NCO group. Indeed, it was methyl isocyanate (CH3NCO) that was responsible for the deaths of thousands in Bhopal, India in 1984.[[#_ftn3|[3]]]However, if appropriate precautions are taken the use of diisocyanates is quite safe and lead to a non-hazardous product like me. My parent diisocyanate is Isophorone Diisocyanate (IPDI). Here is a picture of it.[[#_ftn4|[4]]]
Capping agents are what give urethane acrylate oligomers their UV-reactivity and what makes them different from other polyurethanes.The acrylate group is a C = C double bond adjacent to a carbonyl which undergoesphoto-polymerization.Some can be corrosive and irritating to the skin, but they are not nearly as harmful as the isocyanates. My parent capping agent 2-hydroxyethyl acrylate, which is the most common of the capping agents used. Of course, here is a picture.[[#_ftn5|[5]]]
Part II.My Conception (a.k.a Reaction Mechanism and Conditions) (Principles Implicated: 1, 2, 3, 5, 6, 7, 8, 9 & 11)
Now that I have introduced my parents, it is time to move to where most of the magic happens; the actual making of me. I am fundamentally the product of a urethane reaction: a reaction between an alcohol and an isocyanate group generalized as [[#_ftn6|[6]]]
In the case of urethane acrylate oligomers, the idealized reaction is one end of the diisocyanate reacting with each hydroxyl of the polyol and the other end reacting with the hydroxyl of the capping agent, like so: CA-DI-P-DI-CA This reaction has several environmental advantages.First, as the reaction is essentially just a rearrangement of atoms, it has an atom economy of 100 percent.Nothing goes to waste and even unintended byproducts, such as a CA-DI-CA adduct, will eventually become a part of my adult form. Second, the harmful NCO groups are removed and replaced with the extremely strong urethane linkage making me less toxic than my starting raw materialparents.This can easily be assured through the in-process monitoring of the isocyanate absorbance peak on my FTIR spectra.This peak absorbs at ~2400 cm-1 which is literally out in the middle of nowhere and cannot really be missed. When the peak disappears, the reaction is complete.
FTIR Spectra of Oligomer Containing Free NCO
Third, the urethane reaction is also a very efficient reaction.It is exothermic and, depending on the isocyanate used, goes to completion in a relatively short amount of time.It proceeds at room temperature even without any catalyst.If the reaction time does need to be shortened, the reaction can be catalyzed with amines or organometallic-complexes.The most common catalyst is Dibutyltin Dilaurate (DBTDL), however bismuth or zinc catalysts can also be used and have the advantage of being less toxic than their tin based counterparts. Finally, even the trace chemicals can be ecofriendly.Small amounts of antioxidants are added to prevent my premature and unintended polymerization.One such antioxidant commonly used is Butylated hydroxytoluene (BHT), an FDA approved food additive.
Infant Macro-Me
Molecular Me (formula: C38H62N4O12)
Let's do a double check of my atom economy. I have one molecule of 1,4-butandiol at 90.1 g/mol, two molecules of IPDI at 222.3 g.mol each, and two molecules of HEA at 116.1 g/mol each. My molecular formula is C38H62N4O12 which calculates out to 766.9. So:
[1(90.1)+2(222.3)+2(116.1)]/766.9 = 766.9/766.9 = 1.00 Perfect ;) My FTIR spectra also shows none of the hazardous free NCO present either.
Final Product FTIR without Free NCO
Part III.All Grown Up (The UV cure of an Urethane Acrylate Oligomer) (Principles Implicated: 1, 2, 3, 5, 7 & 9)
The final stage in my life is my metamorphosis from a liquid to a solid via free radical polymerization.The reaction is typically catalyzed by a photoinitiator, such as a peroxide, which decomposes into a highly reactive free radical when exposed to UV light in the range of 254-400 nm.These radicals then react with my acrylate functionality to produce more radicals and the polymerization proceeds in the usual way for unsaturated compounds: chain initiation, chain propagation, and finally chain termination.Like the urethane reaction that produced me, this reaction is exothermic and very fast, sometimes taking less than one minute of UV exposure to complete.It also has a near perfect atom economy as it is merely a rearrangement of bonds between the same atoms. Polymers produced in this way have physical properties that are quite diverse due to the diversity in starting raw materials.Long linear chains with low acrylate functionality tend to give soft, flexible polymers.Short chains and higher acrylate functionality produce more highly branched and cross-linked networks resulting in hard, brittle polymers.
The Life of a Urethane Acrylate Oligomer
In 2005, BASF Corporation won the Presidential Green Chemistry Challenge Award in the Greener Reaction Conditions category for their urethane acrylate oligomer primer system. Among many others, this system has the notable environmental advantages of low energy consumption and low VOC emissions. The purpose of this page is to summarize the “life” of a typical urethane acrylate oligomer, like the one produced by BASF, from start to finish in order to better understand how “green” the system actually is.
Part I. My Parents (a.k.a. Raw Materials)
(Principles Implicated: 2 & 4)
As with most things on the planet Earth, my life begins with my parents; the stuff that went into my creation. In my simplest form, I am made up of three main components: a di- or tri- functional polyol, a difunctional isocyanate, and another hydroxy containing compound which also has the reactive acrylate functionality. This third component is sometimes called a capping agent.
Polyols are rather innocuous, non-hazardous starting materials some of which occur in nature, such as castor oil and sucrose, or can be derived from renewable vegetable oils. They can be aliphatic, aromatic, linear, cyclic, polyester based or polyether based, or any combination of the above. These various polyol characteristics that are inherited by urethane acrylate oligomers leading to enormous versatility in physical properties.
My polyol parent is 1,4 butanediol. Here's a picture.
Unlike polyols, isocyanates are quite a hazardous bunch causing asthma in humans due to the high reactivity of the NCO group. Indeed, it was methyl isocyanate (CH3NCO) that was responsible for the deaths of thousands in Bhopal, India in 1984.[[#_ftn3|[3]]] However, if appropriate precautions are taken the use of diisocyanates is quite safe and lead to a non-hazardous product like me.
My parent diisocyanate is Isophorone Diisocyanate (IPDI). Here is a picture of it.[[#_ftn4|[4]]]
Capping agents are what give urethane acrylate oligomers their UV-reactivity and what makes them different from other polyurethanes. The acrylate group is a C = C double bond adjacent to a carbonyl which undergoes photo-polymerization. Some can be corrosive and irritating to the skin, but they are not nearly as harmful as the isocyanates.
My parent capping agent 2-hydroxyethyl acrylate, which is the most common of the capping agents used. Of course, here is a picture.[[#_ftn5|[5]]]
Part II. My Conception (a.k.a Reaction Mechanism and Conditions)
(Principles Implicated: 1, 2, 3, 5, 6, 7, 8, 9 & 11)
Now that I have introduced my parents, it is time to move to where most of the magic happens; the actual making of me.
I am fundamentally the product of a urethane reaction: a reaction between an alcohol and an isocyanate group generalized as [[#_ftn6|[6]]]
In the case of urethane acrylate oligomers, the idealized reaction is one end of the diisocyanate reacting with each hydroxyl of the polyol and the other end reacting with the hydroxyl of the capping agent, like so:
CA-DI-P-DI-CA
This reaction has several environmental advantages. First, as the reaction is essentially just a rearrangement of atoms, it has an atom economy of 100 percent. Nothing goes to waste and even unintended byproducts, such as a CA-DI-CA adduct, will eventually become a part of my adult form.
Second, the harmful NCO groups are removed and replaced with the extremely strong urethane linkage making me less toxic than my starting raw material parents. This can easily be assured through the in-process monitoring of the isocyanate absorbance peak on my FTIR spectra. This peak absorbs at ~2400 cm-1 which is literally out in the middle of nowhere and cannot really be missed. When the peak disappears, the reaction is complete.
Third, the urethane reaction is also a very efficient reaction. It is exothermic and, depending on the isocyanate used, goes to completion in a relatively short amount of time. It proceeds at room temperature even without any catalyst. If the reaction time does need to be shortened, the reaction can be catalyzed with amines or organometallic-complexes. The most common catalyst is Dibutyltin Dilaurate (DBTDL), however bismuth or zinc catalysts can also be used and have the advantage of being less toxic than their tin based counterparts.
Finally, even the trace chemicals can be ecofriendly. Small amounts of antioxidants are added to prevent my premature and unintended polymerization. One such antioxidant commonly used is Butylated hydroxytoluene (BHT), an FDA approved food additive.
Let's do a double check of my atom economy. I have one molecule of 1,4-butandiol at 90.1 g/mol, two molecules of IPDI at 222.3 g.mol each, and two molecules of HEA at 116.1 g/mol each. My molecular formula is C38H62N4O12 which calculates out to 766.9. So:
[1(90.1)+2(222.3)+2(116.1)]/766.9 = 766.9/766.9 = 1.00 Perfect ;)
My FTIR spectra also shows none of the hazardous free NCO present either.
Part III. All Grown Up (The UV cure of an Urethane Acrylate Oligomer)
(Principles Implicated: 1, 2, 3, 5, 7 & 9)
The final stage in my life is my metamorphosis from a liquid to a solid via free radical polymerization. The reaction is typically catalyzed by a photoinitiator, such as a peroxide, which decomposes into a highly reactive free radical when exposed to UV light in the range of 254-400 nm. These radicals then react with my acrylate functionality to produce more radicals and the polymerization proceeds in the usual way for unsaturated compounds: chain initiation, chain propagation, and finally chain termination. Like the urethane reaction that produced me, this reaction is exothermic and very fast, sometimes taking less than one minute of UV exposure to complete. It also has a near perfect atom economy as it is merely a rearrangement of bonds between the same atoms.
Polymers produced in this way have physical properties that are quite diverse due to the diversity in starting raw materials. Long linear chains with low acrylate functionality tend to give soft, flexible polymers. Short chains and higher acrylate functionality produce more highly branched and cross-linked networks resulting in hard, brittle polymers.
This is me: